In certain types of lamp systems, it is desirable that the lamp produce a non-wavering, or uniform, level of light as soon as the lamp is turned on. This is particularly true in automotive headlamp systems. Certain types of high pressure gas discharge lamps, however, are apt to exhibit a non-uniform level of light intensity during a lamp warm-up period. A metal halide lamp, for instance, includes, within a sealed arc tube, a gaseous mixture including vaporized mercury during steady state lamp operation. When the lamp has just become powered and is warming up from ambient temperature, however, the mercury is still in a liquid state and tends to condense on the inner wall of the arc tube. The condensed mercury tends to block light from being transmitted to outside the arc tube.
In order to provide essentially instant light output from a metal halide lamp, xenon has been incorporated into the gaseous mixture within the arc tube. U.S. Pat. No. 5,239,230 issued to Mathews et al on Aug. 24, 1993 and assigned to the same assignee disclosed such a light source. Additionally, another such light source is disclosed in U.S. Pat. No. 5,059,865 issued to Bergman et al on Oct. 22, 1991 is also assigned to the same assignee as the present invention. However, mercury condensation, as mentioned above, still partially blocks the xenon-generated light from being transmitted to outside the arc tube. When the lamp has eventually warmed up, the mercury becomes vaporized, and undergoes quantum state emissions of electrons to produce light. Typical warm-up periods vary from about 40 seconds for a 60-watt lamp to about 7 minutes for a 400-watt lamp.
Amongst the various attempts in the prior art to assure a uniform level of light from a high pressure gas discharge lamp, one method takes advantage of the fact that lamp voltage increases during lamp warm-up. This voltage increase is due to mercury vaporization. The ballast circuit is designed to respond to lamp voltage, such that the ballast power delivered to the lamp is a decreasing function of lamp voltage. While improving the uniformity of light intensity during lamp warm-up, such method still falls considerably short of providing a highly uniform level of light that is constant to within, for instance, 5 percent.
Another prior method seeking to promote uniformity of light intensity during lamp warm-up uses a dual time-constant circuit, which "remembers" how long the lamp had been on during a prior run (within the immediately preceding 40 seconds) as well as how long it had been off (within the immediately preceding 6 minutes). The ballast applies a start-up power magnitude and decay time that depends on these two parameters. The longer the lamp had cooled, the longer the start-up power is applied and the longer it takes the applied power to decay to steady-state. The foregoing circuit is described in copending patent application Ser. No. 07/858,927, filed Mar. 27, 1992, for "Low Voltage Ballast Circuit for a High Brightness Discharge Light Source," by Joseph M. Allison (the instant inventor) and others. The foregoing application is assigned to the same assignee as the present invention, and its entire disclosure is hereby incorporated by reference. While promoting more uniformity in light intensity during lamp warm-up, however, the foregoing method still falls short of achieving a highly uniform level of light intensity within, for instance, 5 percent uniformity during lamp warm-up.
One possible approach to achieving high uniformity of light intensity during lamp warm-up would be to control the light intensity directly, i.e., by sampling a fraction of the light produced, and feeding back such light information to control circuitry for adjusting the level of power supplied to the lamp, to achieve a more uniform intensity of light. However, this approach, considered by itself, would likely encounter two, significant difficulties. One is that such approach is subject to a wide aberration in the level of light intensity produced, especially over time. That is, over time the percentage of total light intercepted by the feedback light sensor (i.e. the feedback fraction) may change due to accumulation of dirt on optical surfaces, or optical alignment disturbances, for instance. The sensitivity of a feedback control system having high loop gain to the feedback fraction is almost 100 percent. Thus, for example, a 20 percent decrease in the fraction of light sampled results in a 20 percent increase total light. Thus, the foregoing light feedback approach, considered by itself, would likely result in erratically different levels of light produced, especially over time.
A second difficulty with controlling light intensity directly as discussed above, is that an overall darkening of the lamp, from a layer of dirt, for instance, would reduce the amount of light fed back. The feedback circuit, in turn, would attempt to boost the power to the lamp, perhaps well beyond the capability of the lamp ballast, in an attempt to maintain the same level of light output. This would create an unusually large power usage, with potentially destructive consequences to the lamp and its ballast circuitry.
It would thus, be desirable, to provide a ballast circuit for a high pressure gas discharge lamp that utilizes light feedback control during a lamp warm-up period, but that avoids the foregoing problems of producing erratically different light levels as the sampled light intensity varies, and of creating an unusually large power usage for the lamp.